Many existing phase detectors are analog in nature and have a limited dynamic range. Generally, such phase detectors generate an output voltage indicative of the phase difference between two oscillations that are close in frequency. The polarity of the output voltage indicates which oscillation is leading the other. The magnitude of the output voltage tends to be proportional to the phase difference. The dynamic range of such analog phase detectors is typically limited to one cycle in each direction. Digital phase detection is typically preferred for phase detection of dynamic ranges wider than 1 or 2 cycles.
A prior method of phase digitizing that has very wide dynamic range is described in U.S. Pat. No. 5,663,666, entitled DIGITAL PHASE DETECTOR, by Chu and Sommer. Such a method can be used only on a signal operating within a very narrow frequency band, 100 ppm for example, such as a signal from a crystal oscillator. The method also requires a local oscillator operating at near coherence to the signal.
Another prior method of phase digitizing involves time-stamping the zero-crossings of a signal, as described in “Phase Digitizing Sharpens Timing Measurements,” David Chu, IEEE Spectrum, July 1988, pp. 28-32. For precise results, such methods usually involve custom time-digitizer circuits, such as described in U.S. Pat. No. 5,166,959, entitled PICOSECOND EVENT TIMER, by Chu and Knotts. Phase digitizing techniques that involve time-stamping the zero-crossings of a signal are better suited for agile signals of high frequencies, where signal frequencies may change radically and suddenly, and many zero-crossings are available to generate time-stamp data. A penalty for such a wide-band approach is noise.
In an interferometer arrangement, noise is usually generated from fluctuating beam alignment, turbulence, photodiodes, electronic amplification, and the light source itself. In noisy environments, unexpected spurious zero-crossings may occur due to multiple triggering of the same signal edge, causing a catastrophic failure in previous phase digitizing processes.
In metrology of moving objects, signals are generally quasi-sinusoidal and of limited agility due to the physical inertia of objects being monitored. Frequency of the signal is proportional to the velocity of the object being monitored, and phase of the signal is proportional to the distance of travel. Because physical objects cannot instantaneously jump from one velocity to a much different velocity, the frequency of the signals changes relatively slowly.
The frequency of the signal, although changing slowly, may traverse a wide range, including very low frequencies where the number of zero-crossings available for measurement may be at a premium. Also, the occurrences of zero-crossings are generally non-uniform. This non-uniformity may pose additional difficulty in ascertaining the “data age”—the time between event occurrence and the presentation of its measurement data. These factors render the zero-crossing approach not an optimum technique for phase digitizing for interferometry.
A prior method of phase digitizing for interferometry uses block regression as described in U.S. Pat. No. 6,480,126 entitled PHASE DIGITIZER, by Chu, and assigned to Agilent Technologies, Inc. The described method based on linear regression over an entire time segment, and not just at the vicinity of a zero crossing, is effective in averaging out noise. However, the method cannot be used on a signal operating at a frequency within ±100 kHz. This frequency limit effectively places an upper limit on the velocity of the detected object when the object is moving away from the light source to avoid entering this frequency band.
Therefore, there is a need for a phase digitizing system and method that employs digital signal processing for continuously generating noise-suppressed digital phase data representing the phase of an incoming analog signal, without the disadvantages of previous phase digitizing techniques.